Explosive composition



United States iatented Sept. 5, 1961 2,999,014 EXPLOSIVE COMPOSITIDN Hartwell Henry Fassnacht, Woodbury Heights, and Donald Louis Sagers, Woodbury, N.J., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed June 10, 1958, Ser. No. 741,004 3 Claims. (Cl. 52-11) The present invention relates to an improved dynamite composition. More particularly, the present invention relates to a dynamite composition especially suited for use in underground operations.

Conventional dynamite containing nitroglycerin as the principal sensitive explosive ingredient is widely used, particularly in underground operations such as mining, because of its excellent sensitivity, even in small diameter; its stability; its uniformity of action; and its favorable oxygen balance. However, one main disadvantage of conventional dynamite is that its nitroglycerin component causes severe headaches to the persons coming in contact with it or inhaling its vapors, for example, because of leakage of the nitroglycerin from the dynamite cartridge or because of unreacted nitroglycerin in the debris from the blast. This physiological property is not peculiar to nitroglycerin alone but also to all other liquid aliphatic nitric esters.

In attempts made to eliminate this headache problem, other sensitive explosive ingredients have been substituted for the nitroglycerin in dynamite formulations. However, the resultant formulations have not been entirely satisfactory from the standpoint of performance. For example, when the nitroglycerin was replaced by PETN (pentaerythritol tetranitrate) in conventional formulations, their sensitivity, as evidenced by their ability to propagate the detonation over an air gap, was decreased to a substantial degree. Underground boreholes frequently are horizontal or inclined slightly from the horizontal direction. The explosive cartridges must be pushed and tamped into the borehole one at a time. Because of dirt and loose material from the borehole walls, as well as failure on the part of the loader to insert fully each cartridge, gaps between individual cartridges are common. Compositions of low sensitiveness are not satisfactory for this type of shooting because of the likelihood that all of the load in the borehole will not fire if a gap does exist. Because of the foregoing, conventional formulations based on PETN have not met with commercial success.

In order to increase the air-gap sensitiveness of PETN formulations, special measures have been taken in preparing these compositions. As an example, Johnson and Lewis (US. Pat. 1,972,938, September 11, 1934), improved the sensitiveness to some extent and to a variable degree by coating the PETN onto the ammonium nitrate particles prior to their incorporation into the final formulation. However, the dynamites prepared by this special procedure still lacked suflicient sensitiveness and dependability of action for commercial acceptance.

More recently, a dynamite formulation free of nitroglycerin and having acceptable sensitiveness was developed. In this composition, which is described in US. Patent 2,499,321 (Lyte, February 28, 1950), the sensitive explosive component is pentolite, a mixture of PETN and trinitrotoluene. Although the presence of the latter compound in conjunction with the PETN increases sensitiveness to a satisfactory level, it also results in the production of noxious fumes common to lN l-containing compositions. Such fumes of course are highly undesirable in underground workings, and operations must be delayed after the blast, sometimes for substantial intervals, until the fumes and smoke have dissipated. For this reason, fume-producing compositions have been discriminated against in these underground operations and recourse has been made reluctantly to the headache-causing nitroglycerinbased explosives. Obviously, a need exists in the art for an explosive composition having the advantageous features of conventional dynamites without their headache-producing propensities.

Accordingly, an object of the present invention is the provision of a dynamite composition free of headachecausing components. Another object of the present invention is the provision of such a dynamite which does not cause undesirable fumes and thus is suitable for use in underground operation. Further objects will become apparent as the invention is further described.

We have found that the afore-mentioned objects may be achieved when we prepare an explosive formulation in which PETN is the only organic explosive component and is of such particle size that the ratio of the particle size of the inorganic oxidizing salt to that of the PETN is a least 4 to 1. That the ratio of the particle size of the PETN and the oxidizing salt effects the sensitivity of the composition is a surprising and unexpected finding.

In accordance with the present invention, a dynamite composition is provided comprising 720% PETN as the sole organic explosive component, -90% ammonium nitrate, 25% of which may be replaced by sodium nitrate, and 310% combustible material, the ratio of particle size of the oxidizing salt(s) to the PETN being at least 4/ 1.

The following examples serve to illustrate specific embodiments of the composition of the present invention and to demonstrate the criticality of various factors. However, they Will be understood to be illustrative only and not as limiting the invention in any manner. Parts given in the examples are parts by weight.

EXAMPLE 1 In order to demonstrate the effect on sensitiveness of the particle size ratio of ammonium nitrate to PETN, a series of mixings were prepared by using selected screen cuts of ammonium nitrate and of PETN to provide ratios of particle size of these ingredients. In all cases, by weight of ammonium nitrate was thoroughly admixed with 15% by weight of PETN, and the mixtures were packed in 1%. X 8 inch paper shells and tested for airgap sensitiveness. In this test, an explosive cartridge is cut in two at the center and the cut ends are covered with a sheet of tissue paper to prevent spillage of the contents. The two halves are aligned with cut ends facing in a sheet of paper which then is rolled to maintain the halfcartridges in a fixed position. One half-cartridge, termed the primer, is initiated by a standard No. 6 blasting cap and the distance, the air gap, over which the resultant detonation travels to initiate the other half-cartridge, the receptor, is determined by a trial and error method by adjusting the gap between half-cartridges in a series of trials, similar to the method employed in the drop test. The air gap over which the detonation of the primer propagates to initiate the receptor in 50% of the trials is termed the 50% detonations point. The screen cuts and particle size ratios used and the half-cartridge air-gap sensitiveness obtained, as indicated by the 50% detonations point, are summarized in the following table.

1 Smaller value is the number of the Tyler screen through the meshes of which the material passed and larger value is the number of the screen the meshes of which held the material. The figure in parentheses represents the average size of the particles in millimeters.

2 N too fine for screen analysis, maximum particle size 01' 0.005 mm. assumed.

Another ammonium nitrate/PETN formulation (92/8 weight ratio) was prepared, in which no control was exercised over the particle size ratio, the PETN being sprayed on the ammonium nitrate to coat it in accordance with the patented Johnson and Lewis procedure. This formulation had an air-gap sensitiveness, as determined by the afore-described test, of 1 inch (50% detonations point). By controlling the particle sizes such that the particle size ratio of amonium nitrate/PETN exceeded 4/1, an 92/8 (by weight) ammonium nitrate/PETN blend was formulated having an air-gap sensitiveness, in spite of low PETN content, of 5 inches (50% detonations point).

=Inasmuch as the mixtures of Example 1 are not oxygenbalanced and hence are unsuitable for blasting operations, a series of formulations were prepared as described in the following examples, which formulations when packaged in a combustible shell, eg of paper, are essentially oxygen balanced. Moreover, although the previous example clearly indicates the effect of particle size ratio, such compositions which are not oxygen-balanced tend to exhibit a sensitiveness slightly greater than that of corresponding oxygen-balanced formulations, i.e. those containing the same Weight ratio of PETN/ammonium nitrate in the same particle size ratio.

EXAMPLE 2 Into a standard mixer for granular explosive compositions were charged 2160 parts (72% by weight of the mix) of ammonium nitrate of a standard blend which passes through an 8-mesh Tyler screen and substantially held on a 325-mesh screen, 240 parts (8%) of sodium nitrate passing through 8-mesh and held on ZOO-mesh, 420 parts (14%) of PETN having a maximum particle size of 0.005 millimeter, 30 parts (1%) of wood pulp, and 150 parts (5%) of apricot pit meal. The particle size ratio of the mixed oxidizing salts to PETN was at least 12/ 1. The ingredients were mixed at ambient temperature for about minutes, and then the mix was packed into standard 1% x 8 inch paper shells. :The oxygen balance of the composition exclusive of shell was +8.8, the stick count (density as expressed in number of 1% x 8 inch cartridges per 50-pound case) being 137. In the half-cartridge air-gap sensitiveness test, the 50% detonations point was 14 inches.

Another formulation was prepared in accordance with the above procedure with the exception that a commercial blend of coarse PETN was substituted for the fine PETN to obtain a formulation in which the ratio of oxidizing salts particle size to PETN particle size was less than 4/ 1. The resultant formulation had a stick count of 144 and a half-cartridge air-gap sensitiveness" of only 5 inches (50% detonation point) although it contained a large proportion of PETN.

EXAMPLE 3 In accordance with the procedure of Example 2, various formulations of the dynamite of the present invention were prepared by using PETN having a maximum particle size of 0.005 millimeter, ammonium nitrate passing through an 8-mesh Tyler screen and held on a 325- mesh screen, and sodium nitrate passing through 8-mesh and held on 200-mesh, the particle size ratio of the oxidizing salt(s) to PETN being at least 12/1. The compositions and their properties are summarized in Table II.

Table II Mix No 15 l6 l7 18 19 20 Ingredients (percent):

PETN 7. 0 10.0 15.0 15.0 20 7 NH4NO3. 85.0 82. 5 69. (l 50. 0 (i4 NaNOa 10.0 25.0 10 Wood Pulp 1.0 1 Apricot Pit Meal. 8. 0 7. 5 5

Finely Divided Gelatinized Starch Powder...

Rye Flour Resin-impregnated Bagasse "Pith 1 Properties:

Oxygen Balance (Exclusive of Shel1) +6.7 +6.5 +9., +8.4 +8.2 +9.3 Stick Count 13 l 12'} 118 1'2') Half-Cartridge Air- Gan Sensitiveness-50% Detonations point (in). 0 10 11 10 13 7 1 Comprising 2.5% coumurone-indenc resin and 0.5% bagasse pith.

As is evident from the foregoing examples, dynamites of satisfactory sensitiveness, i.e. having air-gap sensitiveness of at least 6 inches (50% detonations point) and containing PETN in economically feasible amounts as the sole organic explosive component can be readily obtained by proper regulation of particle size of the PETN and oxidizing salts. As long as the ratio of the particle size of the oxidizing salts to that of the PETN is maintained at at least 4/1 the desired results are obtained. By reference particularly to Example 1, it may be seen that for the inherently more sensitive ammonium nitrate-PETN compositions which are not oxygen-balanced, the sensitiveness increased to a great extent, i.e. from 6 to 11 inches as determined by the air-gap sensitiveness test when the ratio was increased from 3/1 to 4/ 1, the high degree of sensitiveness being maintained with further increases in this ratio, for example, up to 72/1. This increased sensitiveness is independent of the degree of fineness of the PETN and of the salts. That is, by maintaining the particle size ratio at least 4/ 1, the desired sensitivcness is obtained regardless of whether the salts and PETN are in the form of relatively coarse particles or in the form of relatively fine particles. The measure of particle size represents the maximum dimension of the particle.

Moreover, the particle size of either component need not be uniform, and, thus, commercial grades of the PETN and of the oxidizing salts may be used in the formulations, these commercial grades constituting blends of PETN or of oxidizing salt of various particle sizes. For example, a

combination of mixes Nos. 6 and 7, each mix meeting the requirement of a 4/1 particle size ratio although the particles of mix 7 are larger than those of mix 6, would give a composition having a satisfactory air-gap sensitiveness. This fact is substantiated by Examples 2 and 3, in the formulations of which blends of ammonium nitrate and sodium nitrate were used. Obviously, as long as the average particle sizes of both the PETN and the oxidizing salt or salts used in the formulation are such that the particle size ratio of oxidizing salt or salts to PETN is essentially at least 4 to l, the desired results are obtained. Although we do not wish to be limited by a theoretical discussion, we believe that the fulfillment of this prerequisite provides a continuous matrix of PETN particles through the mixture, which matrix makes possible the heretofore unobtainable sensitiveness of dynamites containing only economically feasible amounts of PETN.

Satisfactory formulations contain PETN in the amount of 720% by weight of the mixture. The use of lesser amounts of PETN results in compositions lacking in sensitiveness, whereas the use of greater amounts of PETN is impractical for economic reasons. As afore-mentioned, both coarse and fine cuts of PETN and blends thereof may be used when the necessary particle size ratio is maintained.

The ammonium nitrate is used in the amount of 75-90% by weight, and up to 25% of the ammonium nitrate may be replaced by an equivalent weight amount of sodium nitrate in the interests of facilitating the establishment of an oxygen-balanced composition and the regulation of density. The oxidizing salt or salts used may be coated with any of the conventional anti-setting agents, used to protect these hygroscopic materials from moisture, without effecting deleterious results with respect to performance of the formulation. Such anti-setting agents include zinc oxide, kieselguhr, gypsum, calcium or aluminum stearate, and the like.

The combustible, or fuel, is present in the amount of 3-10% by weight, the exact amount used being that required to provide the necessary oxygen balance. Suitable fuels include such particulate, solid, carbonaceous materials as the exemplified rye flour, wood pulp, apprieot pit meal, resin-impregnated bagasse pith, and finely divided gelatinized starch powder and also corn flour, ivory nut meal, soybean meal, wheat flour, brewers flakes, oat or rice hulls, walnut meal, ground cork, bagasse pith, barley meal, spelt meal, linseed meal, rosin, natural and synthetic resins, and the like. A mixture of combustibles naturally may be used. The exact combustible used will be determined by many factors including density control, highdensity combustibles such as ivory nut meal, walnut meal, and soybean meal being used to bring about higher density and low-density combustibles such as expanded cereals, wood pulp, and ground cork effecting lower densities.

The formulation may also contain, in addition to the previously anti-setting agents, those additives conventionally used in small quantities to modify the formulations. Such additives are antacids, for example zinc oxide which functions also as an anti-setting agent and calcium carbonate, and paste-formers such as guar fluor or bentonite, materials which swell or form gels in the presence of water and thus prevent contact of the water with the hygroscopic salts of the formulations. Furthermore, such materials may also be present which tend to make the PETN less dusty and more free flowing. When the additive also functions as a combustible, the amount of combustible used in the formulation must be adjusted accordingly.

The dynamite compositions of the present invention have a moisture content (as made) of less than 1%, a density of 0.8-1.2 grams per cubic centimeter, and an oxygen-balance, exclusive of wrapper of +614. The paper wrappers conventionally used have a negative influence upon the oxygen balance, and, therefore, the explosive cartridge comprising the composition within a paper wrapper will have an oxygen balance of approximately zero. As previously stated, the compositions have an air-gap sensitiveness of at least 6 inches (50% detonations point).

The invention has been described in detail in the foregoing. However, it will be apparent to those skilled in the art that many variations are possible without departure from the scope of the invention. We intend, therefore, to be limited only by the following claims.

We claim:

1. A dynamite composition having a density of 0.81.2 grams per cubic centimeter, an oxygen balance of +614, and a moisture content less than 1%, comprising 720% by weight of pentaerythritol tetranitrate as the sole organic explosive ingredient, -90% by weight of ammonium nitrate, and 3-10% by weight of a particulate, solid, carbonaceous fuel, the ratio of the particle size of the ammonium nitrate to the particle size of the pentaerythritol tetranitrate being essentially at least 4 to 1.

2. A dynamite composition according to claim 1, wherein up to 25% by weight of the annnonium nitrate is re placed by an equivalent weight percentage of sodium nitrate, the ratio of the particle size of the sodium nitrate to the particle size of the pentaerythritol tetranitrate being essentially at least 4 to 1.

3. A dynamite composition according to claim 1, wherein the carbonaceous fuel comprises a member of the group consisting of wood pulp, apricot pit meal, rye flour, finely divided gelatinized starch powder, resin-impregnated bagasse pith, and their mixtures.

References Cited in the file of this patent UNITED STATES PATENTS 2,107,157 Lewis et al Feb. 1, 1938 2,171,379 Wahl Aug. 29, 1939 2,602,732 Farr July 8, 1952 

1. A DYNAMITE COMPOSITION HAVING A DENSITY OF 0.8-1.2 GRAMS PER CUBIC CENTIMETER, AN OXYGEN BALANCE OF +6+4. AND A MOISTURE CONTENT LESS THAN 1%, COMPRISING 7-20% BY WEIGHT OF PENTAERYTHRITOL TETRANITRATE AS THE SOLE ORGANIC EXPLOSIVE INGREDIENT, 75-90% BY WEIGHT OF AMMONIUM NITRATE, AND 3-10% BY WEIGHT OF A PARTICULATE, SOLID, CARBONACEOUS FUEL, THE RATIO OF THE PARTICLE SIZE OF THE AMMONIUM NITRATE TO THE PARTICLE SIZE OF THE PENTAERYTHRITOL TETRANITRATE BEING ESSENTIALLY AT LEAST 4 TO
 1. 